1. Field of the Invention
This invention relates to a battery pack incorporating a lithium ion battery, and more particularly to a battery pack in which a circuit board protecting the battery is improved.
2. Related Art
A rechargeable battery pack used for a portable telephone set or the like is a lithium ion battery which is incorporated in an insulating package. The battery pack includes a protective circuit which prevents the over-discharge or over-charge of the battery. A plurality of circuit parts are built on a circuit board having a printed wiring pattern. A battery pack whose miniaturization is advanced, the space for the circuit board is limited. Therefore, a circuit board whose both sides are used is employed; that is, parts are built on both sides of one circuit board.
FIGS. 3(A) and 3(B) are a plan view and a side view of a conventional protective circuit COB (chip on board) type circuit board, respectively. In those figures, reference numeral 10 designates a circuit board whose both sides are used formounting parts (hereinafter referred to as "a circuit board", when applicable); 11, a first part mounting on the front surface side; 12, a second part mounting surface on the rear surface side. Reference numeral 22 designates a circuit part mounted on a second part mounting surface 12. On both part mounting surfaces, the wiring patterns are not shown. Reference numeral 13 designates through-holes which connect the two front and rear wiring patterns.
Some of the circuit parts are resin-sealed for water proof. Those parts are, in general, transistors and ICs (semiconductor integrated circuit devices). In the case FIG. 3(A), the parts 23 and 24 are to be resin-sealed; that is, those parts are covered with resin 30 after being mounted. The parts of this kind; that is, the transistors and ICs 23 and 24 are molded in their packages, and mounted on the circuit board 10 by soldering.
In the case of a battery pack used for a portable telephone set, there is a strong demand for reduction in weight. One method of reducing the weight of the protective circuit shown in FIGS. 3(A) and 3(B) is to use a bare chip of a transistor or IC. In the case of use of the bare chip, partial sealing of the parts with the resin 30 is insufficient in water proof. Especially, in the case where the circuit board 10 whose both surfaces are used for mounting parts is employed, the resin sealing (30) of one surface is low in reliability. On the other hand, in the conventional battery pack, the protective circuit board is laid out in the spaces or gaps left in the pack, so that the layout of the wires is in disorder, and it is impossible to coat the whole circuit board 10 with the resin 30. Those are the problems to be solved by the invention.